The European automotive industry has employed tungsten-halogen incandescent lamps for a number of years and the United States automotive industry has been using them in ever increasing amounts. Their advantages, when compared to the standard sealed beam headlight, are well known: a whiter light is emitted; a smaller size lamp produces an equivalent or even greater quantity of light; the intensity of the illumination remains virtually constant over the life of the lamp; and the service life is significantly longer.
Tungsten-halogen lamps operate at much higher temperatures, however, than conventional incandescent lamps. For example, temperatures in localized areas of the tungsten-halogen lamp envelope may range up to 700.degree. C. Consequently, glasses suitable for such envelopes must be thermally stable (resist devitrification) and withstand thermal deformation at those temperatures.
Envelopes for such lamps have been prepared from 96% SiO.sub.2 glasses of the type typified by Code 7913 glass, marketed by Corning Glass Works, Corning, N.Y., under the trademark VYCOR.RTM.. Those glasses exhibit annealing points in the vicinity of 1000.degree. C. and are essentially unaffected by the temperatures encountered during operation of the lamps.
Nevertheless, because of the very high annealing temperature, it is difficult and expensive to form into shapes and lampwork. Accordingly, a "softer" 96% SiO.sub.2 glass was desired, i.e., a 96% SiO.sub.2 glass exhibiting a somewhat lower annealing temperature, but which would retain the thermal stability and low water content of the glasses currently used for the lamp envelopes. A softer glass would permit more rapid machine sealing operations, less thermal wear of machine parts, and a reduction in energy consumption.
A low water content in the glass is critical in the operation of the lamps. For example, the glass may bubble when flameworked, or the interior surface may become blackened during use due to deterioration of the tungsten filament. Furthermore, removal of water from the glass increases the infrared transmittance of the glass at the well-known absorption band for wavelengths in the region of 2.72 microns. This absorption band typically appears in the infrared transmittance curves of water and of glasses in general and, in the latter, has been attributed to the presence of OH groups in the structure of the glass. Absorption, or conversely transmittance, at a wavelength of 2.6 microns is relatively insensitive to the low concentrations of residual water in glass bodies. Accordingly, the residual water content in glasses is customarily defined in terms of an absorption coefficient which is denominated the "beta value", designated ".beta..sub.OH ", and is calculated from the formula ##EQU1## wherein t=glass thickness in mm
T.sub.2.6 =transmittance in percent at 2.6 microns PA0 T.sub.2.72 =transmittance in percent at 2.72 microns PA0 .beta..sub.OH is expressed in terms of mm.sup.-1 PA0 (a) an article having a desired shape is formed from a parent borosilicate glass; PA0 (b) that article is heat treated at a temperature between about 500.degree.-600.degree. C. for a sufficient length of time to internally separate the glass into a silica-rich phase and silica-poor or borate-rich phase; PA0 (c) the article is contacted with an acid (usually to mineral acid) to leach out the silica-poor phase to produce a porous structure composed of the silica-rich phase consisting of a plurality of intercommunicating, submicroscopic pores throughout the article; PA0 (d) the porous article is washed to remove the leachant residue and dried; and then PA0 (e) the porous article is consolidated into a non-porous body by heating without fusion (generally about 1200.degree.-1300.degree. C). PA0 (1) an article having a particular shape is formed from a parent borosilicate glass; PA0 (2) that article is heat treated to cause the glass to internally separate into a silica-rich phase and silica-poor phase; PA0 (3) the phase separated article is contacted with an acid to leach out the silica-poor phase, yielding a porous structure consisting of the silica-rich phase containing a multiplicity of intercommunicating submicroscopic pores throughout the article; PA0 (4) the porous article is contacted with a fluorine-containing fluid to remove OH groups from the constitution of the glass; PA0 (5) the dewatered article is impregnated with a salt solution containing ions of aluminum, calcium, and potassium and/or sodium; and PA0 (6) the impregnated article is dried and fired without fusion to consolidate it into a non-porous body, thereby incorporating the ions introduced by the impregnating salt. PA0 (a) customarily, the acid leachant will be rinsed out of the pores and the porous body dried before being contacted with the fluorine-containing fluid to dewater the glass; PA0 (b) the fluorine-containing material may be either a liquid or a gas; a material which leaves no non-volatile residue in the pores of the article during or after treatment such as HF, the ammonium fluorides NH.sub.4 F and NH.sub.4 HF.sub.2, and fluorides of paraffin hydrocarbons being preferred; PA0 (c) impregnation with the metal salt solution will generally occur more uniformly when the fluoride-treated structure is wet before the impregnation treatment; PA0 (d) metal salts which thermally decompose to form metal oxides and leave no extraneous non-volatile residue such as Al(NO.sub.3).sub.3.9H.sub.2 O, Ca(NO.sub.3).4H.sub.2 O, KNO.sub.3, and NaNO.sub.3 are preferred; and PA0 (e) rather than first fluoride treating the porous body and then impregnating with the salt solution, the fluorine-containing material may be combined with the salt solution and impregnation carried out in a single step.